1. Field of the Invention
The invention relates to a bipolar transistor.
2. Description of the Related Art
A typical bipolar transistor has an emitter, a base, and a collector. Typically, the emitter and collector are semiconductor materials of one type (N- or P-type), and the base is a semiconductor material of an opposite type (P- or N-type), such that NPN or PNP junctions are formed between the emitter, base, and collector.
When the transistor is activated, a small forward bias voltage is applied between the emitter and the base. The bias voltage lowers the energy barrier that exists at the junction between the emitter and the base, causing the transistor to turn on. When such a junction is made between materials of the same basic composition, the junction is called a homojunction. When such a junction is made between two dissimilar materials, the junction is called a heterojunction.
Lately, practitioners have focused on heterojunction bipolar transistors (HBT) in an attempt to achieve higher switching speeds. For instance, one type of HBT includes an emitter structure formed of N-doped polysilicon and a base layer formed of a P-doped silicon germanium alloy (SiGe).